It’s possible, if Serena Williams goes on to reassert her dominance over the rest of the summer, and caps it with a title in New York, that we’ll look back on her 7-5, 6-0 win over Andrea Petkovic on Saturday as the day she turned her season around.

It hadn’t looked as if that was going to be the case at the start. Like her sister, Venus, on Friday, Serena struggled at first to recover from a hard-fought win the previous night. She rushed between points, was slow to get to balls, was fooled by a few Petkovic serves, and threw in more than her usual share of wild misses. Petkovic didn’t make it easy, either; she used her wide serve to open up the court for her forehand effectively.

The change came at 5-5; Serena said afterward that she thought it was now or never at that stage, and the mindset worked for her. Gone, in a heartbeat, were the errors and tired-looking play. In their place was a patient aggression that led her to finish rallies either at the net, or with her old, unreturnable down-the-line backhand. I don't want to use that overused word "vintage" to describe it, but, well, I can't think of anything else.

There was more where that vintage came from. As each game went by in the second, Serena resembled herself—her best self—a little more. She broke Petkovic at 1-0 with a volley winner, and held at 2-0 and 4-0 with backhand winners; the shot grew both more powerful and more accurate through the afternoon. And her serve, which hasn’t been quite as reliably fierce this season, was a weapon again. Serena won 84 percent of her first-serve points and faced just one break point.

So far the week in Stanford has been just what the tennis doctor ordered for Williams. Yesterday she stayed composed against Ivanovic; today she found her shots against Petkovic. What will she show us on Sunday in the final?